Back in the 80s, you could enlist without a high school diploma and, IIRC, you made it through boot camp, theyd send send you to school to earn a GED and then on to AIT. Todays armed forces are more technical and more selective. Maybe theyre more forgiving and remedial once youre in, I dont know. But you gotta get in and make it through basic before you can begin to really go anywhere.
My nephew says theres an online preliminary ASVAB that you can take to see what the test is like. Well...then take that until you do well on it before taking the real thing, says I. Get used to doing some kind of exercise in the morning, too. The better shape you are going in, the easier it will be.
My brother wanted to join the Marines, but wasn't very good with tests. I bought him some study guides. He passed.
If he's serious about joining and has a recruiter worth a damn, they may have kids already in the DEP (Delayed Entry Program) who have passed and can help him study. The recruiter can also have someone from the DEP PT with your nephew. It will help both get in shape.
I wasn't able to work with my brother (we didn't live anywhere near each other; I was stationed in Lejeune at the time) before he went. He needed to cut about 45lbs and pass the ASVAB in order to sign for the Marines. I called the recruiter and told him what was up. It only took about 2 1/2 months before little brother was ready for MEPs, and another month he was in PI.